1. Field of Invention
The invention relates to a method for simulating the load angle of a polyphase machine from the terminal voltages and line currents of at least two phases of the stator of a polyphase machine which represent the respective stator-oriented a-c variables. The stator-oriented a-c variables are transformed into a first stator current component and a second stator current component of the machine current vector and an amplitude of the machine flux vector which are respective field-oriented d-c variables, wherein the first stator current component lies in the direction of the machine flux vector and the second stator current component is perpendicular to the machine flux vector.
2. Description of the Prior Art
Field-oriented controllers for polyphase machines are commercially available whereby the converter-supplied polyphase machine with the field-oriented control has characteristics which are equivalent to those of a d-c drive statically as well as dynamically. Such a drive, controlled with respect to speed or torque, makes possible a four-quadrant operation, where the machine can be set continuously to the rated torque even at standstill.
The principle of field orientation is based on the fact that the variable terminal voltage and conductor current of at least two phases of the stator of a polyphase machine, measured as a-c variables (stator-oriented variables), are transformed by a field-oriented coorindate transformation into d-c variables in such a manner that a separate control of the magnetizing current (i.sub.1 - control) and the active current (i.sub.2 - control) is possible. A flux control can be superimposed on the i.sub.1 - control and a speed control on the i.sub.2 - control. The generated field-oriented control variables are transformed by a stator-oriented coordinate transformation into stator-oriented control variables and are fed to the converter as control voltages.
From DE-AS No. 28 33 593, a method for simulating the machine flux of a polyphase machine from the terminal voltages and conductor currents of at least two phases of the stator of the polyphase machine is known. As a circuit arrangement for carrying out this method, a computing circuit is provided which simulates the machine flux from the terminal voltages and the conductor currents of two phases of the polyphase machine. This computing circuit is called a voltage model of the polyphase machine.
Furthermore an actual-value computer is known which generates from the terminal voltages and the conductor currents of at least two phases of the stator of the polyphase machine, field-oriented d-c variables, for instance, a first and second stator current component and the amplitude of the machine flux vector. The first stator current component lies here in the direction of a machine flux vector and the second stator current component is perpendicular to the machine flux vector.
For simulating the actual load angle, a quotient is to be formed from the second and first stator current component generated by means of the actual-value computer, from which the arc tangent is calculated. The machine flux vector which is simulated from the terminal variables of the polyphase machine by means of the actual-value computer on which the voltage model is based has an inherent error, the error being noticeable especially in the phase angle of the machine flux vector. Since the first stator current component lies in the direction of the machine flux vector, this error has a full effect on the first stator current component. Thus, the simulation of the actual load angle value has an inherent error.